Typical materials requiring surface flatness at high level include a single-crystal silicon disk called a silicon wafer for producing semiconductor integrated circuits (IC, LSI). The surface of the silicon wafer should be flattened highly accurately in a process of producing IC. LSI etc., in order to provide reliable semiconductor connections for various coatings used in manufacturing the circuits. In the step of polishing finish, a polishing pad is generally stuck on a rotatable supporting disk called a platen, while a workpiece such as a semiconductor wafer is stuck on a polishing head. By movement of the two, a relative speed is generated between the platen and the polishing head while polishing slurry having abrasive grains is continuously supplied to the polishing pad, to effect polishing processing.
As polishing characteristics of a polishing pad, it is requested that a polished object is excellent in planarity and uniformity and a polishing rate is high. The planarity and uniformity of a polished object can be improved to some extent by using a polishing layer having higher elastic modulus. The polishing rate can be improved by increasing the amount of slurry retained on foam having cells therein.
The method of increasing the amount of slurry to be retained includes a method which involves rendering a polishing pad itself hydrophilic, and specific examples include (1) a method which involves introducing hydrophilic groups such as hydroxyl groups into a matrix material and (2) a method which involves mixing a matrix material with a hydrophilic substance. For example, a polishing pad composition comprising (A) a crosslinked elastomer and (B) a substance having functional groups such as hydroxyl groups is disclosed (Patent Literature 1). Further, a polishing tool comprising a hydrophilic substance added to, or (modifying) hydrophilic groups added to, a material constituting the polishing tool is disclosed (Patent Literature 2). In addition, a polishing pad comprising a thermosetting polymer matrix resin containing a hydrophilic and substantially water-insoluble sheet-shaped substance is disclosed (Patent Literature 3). Furthermore, a polishing pad comprising a hydrophilizing agent-containing polyurethane composition containing urethane resin obtained by copolymerizing compounds having hydrophilic groups is disclosed (Patent Literature 4).
In the method (1), however, there is a possibility that when the matrix material is polyurethane, active hydrogen-containing hydrophilic groups such as hydroxyl groups react with isocyanate groups in synthesis of polyurethane, and as a result, an unreacted polyol component remains in the material. This remaining polyol component gives rise to a plasticizing effect so that the physical properties of the polishing pad tend to be deteriorated. In the method (2), the hydrophilic substance is hardly uniformly mixed in the matrix material, so a polishing pad having uniform physical properties cannot be obtained.
When the polishing rate varies from the start of use to the end of use, the polishing conditions should be regulated, and there is also the problem of poor efficiency of polishing.
For example, for the purpose of providing a non-foam urethane polishing material capable of efficiently polishing a semiconductor wafer with excellent planarity, there is disclosed a polishing material comprising a polishing material composition consisting of an isocyanate-terminated urethane prepolymer and an active hydrogen-containing compound, wherein the isocyanate-terminated urethane prepolymer is obtained using an aromatic diisocyanate as polyisocyanate and a polyol component consisting of low-molecular-weight polyol and high-molecular-weight polyol, the low-molecular-weight polyol including diethylene glycol, 1,3-butylene glycol etc. (Patent Literature 5).
For the purpose of endowing a polishing cloth itself with dressing properties to prolong the polishing duration of the polishing cloth, there is disclosed a polishing cloth comprising a polyurethane composition wherein the abrasion wear of the cloth is 150 to 350 mg in a Taber abrasion test (Patent Literature 6).
However, the polishing material disclosed in Patent Literature 5 consists of non-foam urethane, and the non-foam polishing material is provided with grooves because of low polishing rate, but abrasive grains and polishing waste etc. in slurry are topically present etc. to make the stabilization of the polishing rate very difficult. The polishing cloth disclosed in Patent Literature 6 is easily worn, is poor in hardness (due to uneven cells and large cell diameters), and is thus poor in planarity and uniformity, and the polishing rate is inevitably changed significantly.
Further, the conventional polishing pad has a problem of center slow (phenomenon where the center of a wafer is hardly polished).    Patent Literature 1: JP-A 2002-134445    Patent Literature 2: JP-A 2003-11066    Patent Literature 3: JP-A 2002-59358    Patent Literature 4: JP-A 2003-128910    Patent Literature 5: JP-A 2000-17252    Patent Literature 6: JP-A 2001-277101